1. Field of the Invention
This invention relates to microwave propagation and more particularly to an antenna feed network for a radar.
2. Description of the Prior Art
When an antenna of a radar is comprised of a plurality of radiators, it may be desirable to excite the radiators in a selected one of a group of patterns of excitation. When, for example, the radar is used for tracking a target within a spatial region, it is desirable that the antenna provide a narrow, high gain tracking beam. The tracking beam is typically provided by applying excitation to a selected one of the radiators.
When the radar is used for searching for a target, it is desirable that a return signal from the target have a signal strength independent of the altitude of the target. The return signal is independent of the altitude when the intensity of the beam is proportional to the cosecant of the angle of elevation of the target (referred to in the art as a cosecant square beam). Typically, the cosecant square beam is provided in response to excitation being concurrently applied to all of the radiators.
A feed circuit for providing a selected one of a plurality of patterns of excitation may be comprised of what is alternatively known as an orthogonal beam matrix or a Butler matrix. The orthogonal beam matrix provides a pattern of excitation in response to inverse transform signals representative of the discrete inverse Fourier transform of the pattern.
The excitation of one of the radiators is predicated upon the inverse discrete Fourier transform of a unit impulse being a rectangular pulse. Therefore, when all of the signals applied to the orthogonal beam matrix are of equal amplitude, (a discrete signal representation of a rectangular pulse) excitation is applied to a single radiator (a discrete signal representation of a unit impulse). For similar reasons, excitation that causes the cosecant square beam is provided in response to the inverse transform signals being representative of values of a function that is similar to a biased cosine square function.
Heretofore, a simple network for applying alternative excitations to the radiators has been unknown in the art.